Combustion using argon with oxygen

Abstract: A combustion method wherein fuel is mixed with an oxidant mixture comprising oxygen and a defined concentration of argon, and combusted resulting in reduced generation of both NOx and particulate pollutants. Alternatively argon is mixed with fuel and the resulting mixture subsequently mixed with oxidant for combustion.

Description: Technical Field This invention relates generally to combustion carried out in a combustion zone such as an industrial furnace in order to generate heat which may be used to heat a furnace charge. Background Art Many industrial processes employ furnaces wherein fuel and oxidant are combusted to generate heat which is used to heat a charge within the furnace. Among such industrial processes one can name glassmaking wherein the charge is glassmaking materials or molten or solid glass, steelmaking wherein the charge is steel or iron, copper smelting wherein the charge is raw copper and aluminum making wherein the charge is raw aluminum. Heretofore in carrying out such combustion the oxidant employed has been air due primarily to the lower cost and ready availability of air for this use. As is known, air comprises nearly 80 percent nitrogen. The use of air as the oxidant in carrying out combustion may also result in the generation of nitrogen oxides (NOx) which are considered significant pollutants. Accordingly there has arisen a need in recent years to reduce the level of NOx generated by industrial combustion processes. One successful method for carrying out combustion such as industrial furnace combustion is to use pure oxygen as the oxidant for the combustion. This eliminates a large amount of the nitrogen molecules, that might otherwise be available, from the combustion reaction and thus significantly reduces the amount of NOx generated on an equivalent heat generation basis. DE-A 2 360 873 discloses a combustion furnace, into which fuel is provided and combusted with a mixture of oxygen and exhaust gas or with a mixture of oxygen and an inert gas, preferably carbon dioxide. In experiments made to determine the most preferable composition of said mixture, an oxygen concentration of 15 % was found to be most preferable. As oxygen source a high pressure oxygen bomb, a low pressure oxygen container or an oxygen container in combination with pressure means may be used. According to US-A-3 982 878 hydrogen contained in a gaseous mixture of hydrogen and oxygen or air is burned at a controlled and relatively low burning rate in a combustor by regulating the quantitative ratio of oxygen to hydrogen in said gaseous mixture to the stoichiometric ratio, adding a gaseous diluent to said gaseous mixture outside the combustor, and feeding the diluted mixture to the combustor. One problem with combustion carried out with pure oxygen as the oxidant is a reduction in the momentum of the flame or combustion reaction within the industrial furnace. Combustion reaction momentum is important for ensuring the distribution of the heat generated by the combustion reaction throughout the furnace or combustion zone. This is especially the case where the furnace contains a charge which is intended for heating. As is known, momentum is the product of mass and velocity. The reduction in the momentum of a combustion reaction wherein pure oxygen is the oxidant from that of a combustion reaction wherein air is the oxidant results from the fact that an oxidant of pure oxygen contains only about one-fifth of the mass of an oxidant of air on an equivalent oxygen molecule basis. The problem of low combustion reaction momentum with an oxidant of pure oxygen has been successfully addressed by injecting the oxidant into the combustion zone at a high velocity. The high velocity compensates for the reduced mass to maintain the momentum at the desired level. Moreover, the high oxidant velocity creates an aspiration effect wherein furnace gases such as carbon dioxide and water vapor, are aspirated into the high velocity oxidant prior to the mixing of the oxidant with the fuel and the subsequent combustion. This increases the mass of the oxidant fluid and further improves the momentum of the combustion reaction. This significant improvement in the combustion field is disclosed and claimed in U.S. Patent No. 4,378,205 - Anderson and U.S. Patent No. 4,541,796 - Anderson. High velocity oxygen injection, though solving the significant NOx problem of industrial furnaces, is not problem-free. Especially in the case where the furnace contains a charge which may generate significant particulates, such as in the case of glassmaking, the high velocity causes an increased level of particulates to be'generated from, for example, the charge, and carried out of the furnace. This increases the level of particulate emissions of the industrial process and generally requires the addition of costly pollution control equipment to alleviate the problem. Accordingly it is an object of this invention to provide a combustion method which can effectively overcome both the NOx and the particulate pollutant problems discussed above. Summary of the Invention The above and other objects which will become apparent to one skilled in the art upon a reading of this disclosure are attained by the present invention which is: A method for carrying out combustion as defined in claim 1. Brief Description of the Drawing The sole Figure is a stylized, simplified plan view representation of one embodiment of the invention as it might be practiced in conjunction with a cross-fired glassmelting furnace. Detailed Description In the practice of this invention oxidant mixture and fuel are provided into a furnace or combustion zone. The oxidant mixture and fuel are injected together into the furnace through a pre-mixed burner. The invention may be practiced in conjunction with any type of furnace. The invention will have particular utility in the operation of an industrial furnace which contains a charge which is intended to be heated and/or melted, particularly a charge which may generate significant levels of particulate emissions. Specific examples of such furnaces include glassmelting furnaces, steel reheating furnaces, copper smelting furnaces and aluminum production furnaces. The fuel may be any fluid fuel such as methane, propane, natural gas or fuel oil. The oxidant mixture comprises oxygen and argon. Argon is present in the oxidant mixture in a concentration within the range of from 2 to 80 volume percent, preferably within the range of from 2 to 20 volume percent, most preferably within the range of from 3 to 10 volume percent. Preferably the balance of the oxidant mixture is essentially oxygen. By this it is meant that the oxidant mixture comprises only argon and oxygen with small amounts of nitrogen, carbon dioxide, water vapor and/or trace elements generally found in air. The oxidant mixture may be made up in any suitable and effective way. One preferred method for generating the oxidant mixture useful in the practice of this invention is to mix commercially pure argon with technically pure oxygen, i.e. a fluid having an oxygen concentration of at least 99.5 mole percent. The oxygen and argon may be taken from cylinders or from cryogenic liquid tanks. Another preferred way for generating the oxidant mixture useful in the practice of this invention is to pass air through a membrane separation system or a pressure swing adsorption separation system and to operate such system in a manner so as to separate the nitrogen from the oxygen and argon in the air. A particularly preferred pressure swing adsorption system is a vacuum pressure swing adsorption system. Those skilled in the art of air separation are familiar with these terms and with their meanings. The oxidant mixture and fuel combust within the combustion zone to generate heat. The oxidant mixture and the fuel are provided together in a mixture into the furnace or combustion zone, they further mix within the combustion zone; they may already be combusting as they are being provided together into the combustion zone. The use of the argon-based oxidant mixture simultaneously solves both the NOx problem and the particulate problem heretofore experienced in industrial combustion. As is the case with combustion using technically pure oxygen, NOx generation is significantly reduced with the practice of the invention over that resulting from air combustion because very little nitrogen is brought to the combustion reaction with the oxidant mixture. However, because the molecular weight and the density or specific gravity of argon significantly exceeds that of oxygen, the oxidant mixture of this invention has a mass which is significantly greater than would be the case with a pure oxygen oxidant. This increased mass enables one to inject the oxidant mixture into the combustion zone at a lower velocity than would otherwise be the case and still achieve good combustion reaction momentum. The oxidant mixture is provided into the combustion zone at a velocity within the range of from 7.6 to 129.5 m/s (25 to 425 fps), preferably within the range of from 7.6 to 99.0 m/s (25 to 325 fps). Moreover, the increased mass may serve to improve the degree of mixing between the oxidant mixture and the fuel, enabling more complete combustion than might otherwise be the case and reducing the level of products of incomplete combustion, such as carbon monoxide and hydrocarbons, released to the atmosphere. As an added benefit, since argon is an inert gas, deleterious reactions within the flame or combustion reaction, or with the charge if a charge is present in the furnace, are avoided. Still further, the inert argon acts as a heat sink to adsorb some of the heat of the combustion reaction. Since NOx formation is kinetically favored by high reaction temperatures, this further reduces NOx formation such as from infiltrating air or fuel nitrogen, over that which would be generated by combustion with pure oxygen as the oxidant. The minimum oxidant mixture velocity necessary for carrying out the invention is calculated using the following equation: V min = 15.2 m/s•100/(100 + 1.25 A) [V min = 50 fps•100/(100 + 1.25 A)] where A is the concentration of argon in the oxidant mixture. The Figure is a stylized representation of a furnace intended to illustrate a mode of operation of the invention. Referring now to the Figure, there is shown furnace 1. According to the subject invention oxidant mixture and fuel are combined in a premixed burner and passed as a mixture 7 into furnace or combustion zone 1 wherein they further mix and combust. The resulting combustion reaction generates heat and combustion reaction products such as carbon dioxide and water vapor. The heat may be used to heat and/ or melt the charge. In the Figure the charge 8 is molten glass and glass forming materials which pass through the furnace underneath the combustion reactions. The combustion reaction products are passed out of the furnace through exhaust port 9.

Claims: A method for carrying out combustion comprising: (A) providing fuel (2) into a combustion zone (1); (B) providing oxidant mixture (3) into the combustion zone at low velocity, said oxidant mixture consisting essentially of oxygen and argon wherein argon is present in the oxidant mixture in a concentration within the range of from 2 to 80 volume percent; (C) mixing said fuel and said oxidant mixture within the combustion zone; (D) combusting said fuel with said oxygen within the combustion zone; wherein the said low velocity is within the range of from V min to 129.5 m/s (V min to 425 feet per second) where V min is calculated by the formula: V min = 15.2 m/s • 100 100 + 1.25 A where A is the concentration of argon in volume percent in the oxidant mixture, and wherein the oxidant mixture and the fuel are provided into the combustion zone (1) together (7) through a pre-mixed burner. The method of claim 1 wherein the oxidant mixture (3) has an argon concentration within the range of from 2 to 20 volume percent. The method of claim 1 wherein the oxidant mixture (3) has an argon concentration within the range of from 3 to 10 volume percent. The method of claim 1 wherein the combustion zone (1) contains a charge (8). The method of claim 1 wherein the charge (8) comprises glass. The method of claim 1 wherein the said low velocity is within the range of from 7.6 to 99.0 m/s (25 to 325 feet per second).

Inventors: FRANCIS ARTHUR WELLINGTON, FRANCIS, ARTHUR WELLINGTON
Applicants: PRAXAIR TECHNOLOGY INC, PRAXAIR TECHNOLOGY, INC.
Application year: 1994
Main classification: F23L7
IPC codes: C03B5, F23L7